Implications for applicability of the photodegradation and self-recovery of green-emitting CsPbBr3 perovskite nanocrystals

CsPbBr3 nanocrystals (NCs) synthesized by the conventional hot-injection method are photochromatic luminescent nanomaterials due to the photoinduced desorption and re-adsorption of the surface ligands. The apparent color and the luminescence intensity were changed significantly during excitation light irradiation and following dark storage; however, the emission wavelength was almost retained. This work investigates the change in emission color of light-emitting diode lighting using the CsPbBr3 NCs to realize photochromatic luminescence. The results showed definite shifts in emission color caused by changes in optical absorption and green luminescence intensity of the NCs, potentially broadening the application feasibility of CsPbBr3 NCs as photochromatic luminescent nanomaterials.


Introduction
3][4][5][6] Surface ligands adsorbed on CsPbX 3 NCs have a signicant impact on colloidal stability, structural stability, and optical properties. 7,80][11] When the dried solid sample was irradiated with excitation light, the color changed from yellow to black and the photoluminescence (PL) intensity decreased at the same time.The color and PL intensity of the degraded sample naturally returned to the initial state during the dark storage aer the excitation light irradiation was stopped.Based on the Fourier-transformed infrared (FT-IR) absorption measurement, the photodegradation is attributed to the generation of surface defects due to photo-induced desorption of the surface ligands, while the self-recovery in the dark is attributed to surface passivation due to their re-adsorption.The change in PL lifetime also occurred parallel to the change in the FT-IR absorption.The PL lifetime was shortened by photo-excitation, which is attributed to the non-radiative relaxation through surface defects generated by photoexcitation.In contrast, the PL lifetime was completely recovered during the dark storage, which is attributed to the elimination of surface defects by surface passivation. 9n the eld of perovskite-based photovoltaics, rapid selfrecovery of photodegraded polycrystalline perovskite layers within 1 min has been reported.Its mechanism could be explained by ion migration. 12,13This phenomenon is too fast and would make its own application difficult.In contrast, the self-recovery of photodegraded CsPbBr 3 NCs occurred slowly due to re-adsorption of surface ligands.This is a distinguished phenomenon for the NCs adsorbed with surface ligands; it would possibly realize new applications due to the easier recognizability by humans.
One of the main features of CsPbX 3 NCs is that the halogen composition controls the bandgap and the emission color.The self-recovery aer photodegradation was observed even when Br was partially replaced by Cl or I. 11 The change in sample color under excitation light is photochromism.Photochromic materials have a wide range of potential applications in molecular switching, detection, biology, and molecular mechanics. 14hotochromism is generally caused by structural changes in organic dyes.Many hybrid materials with inorganic crystals have been reported; 14 however, there are only few reports on inorganic nanomaterials such as Ag nanoparticles and Cudoped ZnS. 15,16Hybrid photochromic materials based on CsPbX 3 perovskite have also been investigated; photochromism was intrinsically realized by structural changes of organic molecules. 17,18The photochromism of CsPbX 3 NCs that we have been investigated on is due to the formation and decrease of surface defects, which is an unusual phenomenon with a very different mechanism.Our photodegradation and self-recovery cause a large decrease and recovery of PL intensity, while the emission wavelength does not change much.Photochromic PL switching is a technology that has attracted much attention in recent years. 19If not only the sample color but also the emission color can be changed simultaneously, it is expected to be used for a wider variety of applications.We therefore focused on mixed light color of the green PL of the NCs and blue excitation light.
In this work, a nanocomposite lm of CsPbBr 3 NCs was placed on a blue LED to fabricate a lighting device.The light observed from this device is a mixture of green PL and transmitted blue from the LED.The apparent emission color is thought to change with photodegradation and self-recovery of the CsPbBr 3 NCs in the green PL layer.Furthermore, a white lighting device was fabricated using a red phosphor, K 2 SiF 6 :Mn 4+ (KSF), which is a practical material used in wide color gamut displays. 20,21Changes in the luminescent color of the lighting devices were evaluated during continuous exposure to blue light and during the subsequent dark storage without LED illumination.

Preparation of Cs-oleate precursor
11]22 Cs 2 CO 3 (2.50 mmol) was added to a mixture of 1-octadecene (40 mL) and OA (2.5 mL).The mixture was dried for 1 h at 120 °C and then heated under Ar gas to 150 °C.The resulting Cs-oleate precursor solution was preheated to 100 °C.

Synthesis of CsPbBr 3 NCs
A mixture of 1-octadecene (5 mL) and PbBr 2 (0.376 mmol) was vacuum dried for 1 h at 120 °C.Aer purging with Ar gas, OAm (1.0 mL) and OA (1.0 mL) were added to the mixture under stirring at 120 °C, followed by heating to 180 °C.The preheated Cs-oleate precursor (0.8 mL) was rapidly injected to this solution under stirring at 180 °C.Aer aging for 10 s to obtain CsPbBr 3 NCs, the resulting dispersion was rapidly cooled in an ice-water bath for 1 min to end the reaction.Aer adding tertbutanol (25 mL), the aggregated NCs were collected by centrifugation at ∼19 000×g (13 000 rpm using a rotor with a radius of 10 cm) for 5 min.The supernatant was discarded.Aer vacuum drying for 24 h, solid sample was obtained.A toluene dispersion was prepared by dispersing the solid sample under stirring and ultrasonication.

Fabrication of green-emitting and red-emitting lms
Fluorescent lms were prepared using EVA resin. 22Granular EVA (1.0 g) was completely dissolved in toluene (10 mL) mixed with 1-octadecene (0.5 mL), OA (28 mL), and OAm (14 mL) under vigorous stirring for 3 h.Aer Ar gas bubbling at 300 mL min −1 for 5 min, the dried NCs sample (20 mg) was dispersed under ultrasonication and stirring for 5 min.A part of the obtained dispersion (3 mL) was dropped onto a Petri dish (∼58 mm bore diameter) and dried for 1 day under ambient conditions to fabricate a green-emitting CsPbBr 3 NCs nanocomposite lm.A red-emitting lm was prepared in the same way; a commercial KSF powder (0.40 g) was used instead of the solid sample of CsPbBr 3 NCs (20 mg).A part of the prepared suspension (8.0 mL) was dried in the Petri dish aer ultrasonication and stirring to obtain a red-emitting KSF lm.A blank control EVA lm without phosphor was also fabricated.The resulting lms (thickness: 0.1 mm) were cut into a square (20 mm × 20 mm).

Combination of the lms with a blue LED device
Each CsPbBr 3 NCs lm was sandwiched between soda-lime glass plates (20 mm × 20 mm × 1 mm) to shut out the ambient air.11]22 Adhesive aluminum tape was wrapped around the edges to secure the glass plates.The lm samples were placed on a at-panel blue LED (BTE-4556, Nissin Electronics) equipped with a power supply (LPR-10W, Nissin Electronics) and irradiated for 72 h by turning the device on.The luminescence wavelength and irradiance of used blue LED were 468 nm and 48.5 W m −2 , respectively.Aer the irradiation was stopped, they were stored in the dark without separation.The blue LED was temporarily turned on when photography and measurements were needed.

Characterization
The particle morphologies were observed using a eld-emission transmission electron microscope (Tecnai G 2 , FEI).Transmission electron microscopy (TEM) sample was prepared by vacuum drying a drop of NCs dispersion on carbon-reinforced collodion-coated copper grids (COL-C10, Oken Shoji) overnight.X-ray diffraction (XRD) proles were acquired with an Xray diffractometer (MiniFlex600, Rigaku) equipped with a Cu Ka radiation source.FT-IR absorption spectra of dried NCs samples in pressed KBr disks were recorded using an FT-IR spectrometer (FT/IR-4200, JASCO) under an N 2 gas ow of 400 mL min −1 .UVvis absorption spectra were measured using UV/visible/nearinfrared optical absorption spectrometers (V-750 and V-570, JASCO).Here, integrating sphere (ISF-513, JASCO) was used for lm samples to obtain total transmittance and absorbance.Tauc plots were prepared according to the eqn (1) to determine the E g values of the NCs.
where a is absorbance, h is the Planck constant, n is frequency, and A is a constant.The value of n was 0.5 because CsPbBr 3 is a direct transition-type semiconductor.PL spectra were measured using uorescence spectrometers (FP-6500 and FP-8500, JASCO).The obtained spectra were calibrated.Absolute PL quantum yield (PLQY) was determined using a quantum efficiency measurement system (QE-2000-311C, Otsuka Electronics).Emission spectra of the LED devices were measured by an LED evaluation system (Hamamatsu Photonics, C9920-22).

Results and discussion
3.1 Fundamental evaluation of the synthesized CsPbBr 3 NCs The average size of the as-prepared CsPbBr 3 NCs was 8.7 ± 1.7 nm (see Fig. S1 †).They had cubic crystal structure and were modied by oleate anions and oleylammonium cations (see Fig. S2 †).The resulting NCs dispersed in toluene had 2.43 eV of band gap and showed green luminescence under UV excitation (see Fig. S3 †).The band gap was larger than 2.00 eV of bulk cubic CsPbBr 3 , 1 indicating the quantum size effect.

Change in emission color of the LED device attached with the green-emitting lm through the photodegradation and self-recovery of CsPbBr 3 NCs
As illustrated in Fig. 1, to prevent air oxidation for CsPbBr 3 NCs in EVA, the green-emitting nanocomposite lm cut into 20 mm squares was sandwiched between glass plates and xed with aluminum tape.It was placed on a 468 nm blue LED device (48.5 W m −2 ).The nanocomposite lm was irradiated for 72 h and then stored in the dark for 168 h.As shown in the photographs, blackening of the NCs nanocomposite lms occurred during the blue LED illumination, which was accompanied by a decrease in PL intensity.Increase in the PL intensity was observed during storage in the dark.In our previous works, the change in adsorption state of the surface ligands was evaluated by FT-IR analysis on dried solid NC sample. 9,11Unfortunately, effective analysis could not be performed due to the low concentration of NCs in the present nanocomposite lm.The luminescence of the NCs nanocomposite lm placed on the blue LED was also captured.Blue-green light was observed, which was a mixture of green PL of excited NCs and transmitted blue light.The mixed light intensity exhibited decrease and increase in the same manner.Its color also changed to the naked eye, although it is difficult to recognize in the photographs due to performance limitations of the used camera.the initial state.These changes reproduced the results of our previous study. 22The change in emission spectrum is also shown (Fig. 2B).The decrease in blue emission at 468 nm is attributed to increased absorption of the nanocomposite lm becoming black.The decrease in green emission is due to photodegradation of the NCs caused by the photo-induced desorption of surface ligands.Aer the LED was turned off and stored in the dark for 168 h, the emission intensity naturally increased.The blue light intensity also recovered to the initial value as the lm color returned to its initial state.On the other hand, the green PL intensity recovered to only half of its initial value.This is presumably due to the irreversible degradation of NCs gradually progressed by residual oxygen and water molecules in the lm during the 10 days experiment. 22,23egradation occurs without excitation light (see Fig. S4 †).When the nanocomposite lm was stored in the dark, natural degradation was observed.Its absorption spectrum was mostly preserved, while its PL intensity decreased to 43% of the initial intensity aer the dark storage for 10 days.The peak position of green PL shied from 524 nm to 519 nm by light irradiation, and then returned to 524 nm aer storage in the dark.These indicate the desorption and re-adsorption of the surface ligands.The synthesized CsPbBr 3 NCs are quantum dots, as indicated by the quantum size effect described earlier.The band gap of quantum dots is known to change due to the inuence of surface ligands, 24 and it has been reported that the band gap of CsPbI 3 NCs also changed due to surface ligands. 25Therefore, the shis of the PL peak suggest changes in the adsorption state of the surface ligands.Moreover, the NCs in nanocomposite lm are surrounded by EVA resin.EVA, which has ester group (-COO-), possibly affect the photodegradation and self-recovery through adsorption to the NC surface.Unfortunately, effective analysis could not be performed due to the low concentration of NCs in the nanocomposite lm.The chromaticity coordinates shied in accordance with the change in the emission spectrum (Fig. 2C).While the blue LED was turned on, the emission changed from green to blue.Aer the light was turned off, the color shied toward green.If the resin shut out oxygen and moisture, a complete self-recovery of the PL properties of the NCs should occur, 9 resulting in a color shi closer to the initial one.

Change in emission color of the LED device attached
with the red-emitting lm and green-emitting lm through the photodegradation and self-recovery The red-luminescent lm was prepared by dispersing the commercial KSF phosphor to the EVA resin.As shown in Fig. 3A, the KSF lm showed a light-yellow color under white light and red PL under UV light excitation.The transmission spectrum showed an absorption peak of KSF phosphor at 468 nm (Fig. 3B), while the PL spectrum showed multiple sharp peaks due to Mn 4+ at 600-650 nm (Fig. 3C). 20,21As shown in the illustration (Fig. 3D), a white LED device was constructed by overlaying the nanocomposite lm and the KSF lm onto the blue LED.It should be noted that the nanocomposite lm was placed uppermost to directly observe the color change without disassembly.These lms were xed to the integrating sphere attachment, and the LED was turned on continuously for 72 h, and then turned off and stored in the dark for 168 h.The LED was temporarily turned on and off when necessary for photography and emission spectrum measurements.As shown in Fig. 4 and  5, the observed light became greenish white aer 24 h of exposure to blue light and bluish white aer 72 h.Aer 48 h of storage in the dark, the light color turned reddish, and aer 72 h, it became bluish again.Aer 168 h, it showed the same white illumination as the initial state; the chromaticity coordinate of the last emission was located near the rst coordinate.In the photographs when the LED was turned off, the nanocomposite lm turned black by blue light irradiation and returned to its original color during subsequent storage in the dark.
In the emission spectrum of the white LED device (Fig. 4C), the emission peaks of the blue LED, CsPbBr 3 NCs and KSF were observed at 472 nm, 519 nm and 631 nm, respectively.A decrease in the intensity of the blue and red peaks was observed during the LED irradiation (see also Fig. S5 †), caused by increased absorption by the blackened lm.The green peak intensity temporally increased up to 24 h aer blue light irradiation and then decreased.The PL enhancement would be attributed to the photo-activation caused by optimization of the adsorption states of the surface ligands. 26,27This phenomenon should occur in competition with the photo-induced desorption of the surface ligands.When the excitation light intensity is sufficiently strong, the photo-induced desorption becomes dominant and only degradation of PL properties is observed as described earlier.On the other hand, in this experiment, partial blue light transmitted through the KSF lm was irradiated onto the nanocomposite lm.In other words, the nanocomposite lm was irradiated with blue light that was weaker than that directly irradiated from the LED.The PL enhancement due to the photo-activation phenomenon was probably observed as a result of the slower progression of the photo-induced desorption.The increase was temporary, as inuence of the photo-induced desorption became dominant thereaer.The green peak intensity continued to decrease immediately aer the blue LED was turned off (Fig. 4D).However, it began to increase 24 h aer the dark storage began.Slow spontaneous degradation due to residual oxygen and moisture in the lm should progress during the experiment.The effect of readsorption of surface ligands increased gradually, and selfrecovery was observed aer a while.The intensity of the blue

Summary
The demonstration of photochromic PL switching shown by this work would expand the potential applications of photodegradation and self-recovery of CsPbBr 3 NCs.The current major problem is irreversible degradation due to oxygen and moisture. 22,239][30] Another current problem is controlling the speed of recovery, as the full recovery took several months. 9Aer solving the problem, the feasibility of entering new applications in the elds of nanomaterials and LED technology will be demonstrated in the future.Sciences.We thank Ms Y. Nakamura for her experimental assistance.
Fig. 1 (A) Changes in sample color under white light and luminescence under UV excitation for the CsPbBr 3 NCs nanocomposite film after 72 h blue LED irradiation and subsequent 168 dark storage.(B) Emission of the film irradiated with the 468 nm blue LED is also shown.(C) Illustration of the cross-sectional structure and photographs of the settings.

Fig. 2 (
Fig. 2 (A) Change in transmission spectrum for the CsPbBr 3 NCs nanocomposite film.(B) Change in emission spectrum of the nanocomposite film on the 468 nm blue LED device.(C) Corresponding chromaticity coordinates of the emission.

Fig. 3 (
Fig. 3 (A) Photographs under white light and UV excitation, (B) transmission spectra, and (C) PL spectra of the CsPbBr 3 NCs nanocomposite film and KSF film used in a white LED device.(D) Illustration of the cross-sectional structure and photographs of the settings.

Fig. 4 (
Fig. 4 (A) Change in emission color of the white LED composed of the CsPbBr 3 NCs nanocomposite film, KSF film, and blue LED.(B) Photographs under white light with the blue LED turned off are also shown.Corresponding change in emission spectrum during (C) the continuous irradiation and (D) the subsequent dark storage.

Fig. 5
Fig. 5 Change in color chromaticity corresponding to the emission spectrum in Fig. 4.